The present invention relates to microwave delay lines. It also relates to travelling wave tubes having such lines.
As is known the delay line associated with a travelling wave tube functioning as an amplifier ensures the interaction between an electron beam, focussed in accordance with the line axis, and the fundamental transmission mode of the ultra-high frequency travelling wave passing along the line, with a phase velocity close to that of the beam. The electrons transfer energy to the ultra-high frequency wave and there is then an amplification of the ultra-high frequency energy circulating on the line.
At present helical delay lines are used in travelling wave tubes. The disadvantage of these lines in that they bring about the oscillation on the inverse mode of travelling wave tubes for low peak power levels.
Therefore delay lines with a more complicated construction are used and for which the amplitude of the wave transmission inverse mode which causes the oscillation of the travelling wave tube is reduced compared with that of a helical line. Thus, these lines less easily bring about the oscillation of travelling wave tubes and travelling wave tubes functioning as amplifiers with high peak powers can be obtained. It is thus possible to use double helix lines, constituted by two identical helixes of the same longitudinal axis which constitutes the axis of the line, said helixes coming from a same point and having opposite winding directions. Ring and bar or ring and loop lines are also widely used. They are formed by rings, arranged perpendicularly with respect to the transmission direction of the electron beam constituting the axis of the line, two successive rings being connected by a bar parallel to the axis of the line (ring and bar) or by a loop (ring and loop) and two successive bars or two successive loops are diametrically opposed with respect to the rings.
The problem which the present invention aims at solving occurs when the double helix delay lines or the ring and bar and ring and loop lines, which make it possible to increase the HF peak power of travelling wave tubes functioning as an amplifier to which they are connected, are welded, generally by brazing to rods made from a dielectric and good heat conducting material and which are positioned parallel to the axis of the line.
It is known to braze microwave delay lines to rods made from a dielectric and good heat conducting material arranged parallel to the line axis and whose face opposite to that which is brazed to the line is brazed to the sleeve containing the line. The brazing of the rods improves the dissipation of heat between the line and the sleeve and the average power of travelling wave tubes having rods brazed to the line and the sleeve can be approximately ten times higher than that of tubes having rods which are only secured between the line and the sleeve.
When the double helix delay lines of the ring and bar and ring and loop types are brazed to rods, the distance between two successive brazing points being substantially half that existing in the case of a helical line ensuring the same phase velocity of the ultra-high frequency travelling wave, the HF average power of the tubes using these lines is increased, but the HF peak power thereof is limited because breakdowns due to a too strong electrical field occur along the rods between two successive brazing points for a peak power which can be reached by these tubes. The problem which then occurs is that of limiting the peak power of these tubes. This problem is made even greater because it is in practice impossible to forecast the peak power value which brings about the breakdown of the rods due to the irregularity of the brazing points obtained. Thus, on brazing the rods to the line the brazed seam is differently distributed from one case to the next and the breakdown voltage of the rods between two brazed seams is consequently subject to considerable variations.